Numerous nucleating and clarifying agents are used as plastic additives. Such compounds assist in optically clarifying plastics or otherwise improving the processing or physical characteristics of polymers in plastic products. Many plastic products sold and used are made from polymer materials that contain nucleating or clarifying agents within the polymer.
The use of clarifying agents to reduce haze in articles manufactured from crystalline polyolefin resins is well known in the art. Representative acetals of sorbitol and xylitol, which have been employed as clarifying agents, are disclosed generally in the following patents:    Hamada et al., U.S. Pat. No. 4,016,118, dibenzylidene sorbitols    Kawai, et al., U.S. Pat. No. 4,314,039, di(alkylbenzylidene) sorbitols    Mahaffey, Jr., U.S. Pat. No. 4,371,645, diacetals of sorbitols having at least one chlorine or bromine substituent    Kobayashi, et al., U.S. Pat. No. 4,532,280, di(methyl or ethyl substituted benzylidene) sorbitols    Rekers, U.S. Pat. No. 5,049,605, bis(3,4-dialkylbenzylidene) sorbitols including substituents forming a carbocyclic ring.
Co-additives have been used in combination with such clarifiers based on diacetals of sorbitol and xylitol to improve certain properties, such as:    Mentink, U.S. Pat. No. 6,673,856: discloses co-additives to improve the flow behavior and thermal stability of clarifiers based on diacetals of sorbitol and xylitols;    Kobayashi, et al., U.S. Pat. No. 6,245,843 discloses using certain “binders” to depress the melting temperature, improving the dispersion, and improving the flow property of clarifiers based on diacetals of sorbitols and xylitols;
Among clarifiers based on diacetals of sorbitol and xylitol, MILLAD® 3988 clarifying agent is a commercially successful clarifier for polypropylene. It is a dimethyl substituted dibenzylidene sorbitol (“DMDBS”), 1,3:2,4-bis(3,4 dimethyl benzylidene sorbitol). MILLAD® 3988 clarifying agent is manufactured and distributed by Milliken & Company of Spartanburg, S.C.
MILLAD® 3988 clarifying agent can achieve very good optical properties in polypropylene (“PP”) at its optimum loadings, for example, at loadings near 2500 parts per million (“ppm”). However, its clarifying function is not as great when the loading is relatively low, such as 1000 ppm, for example. This is generally true of most clarifiers based on diacetals of sorbitol and xylitol. Though many prior art references have disclosed using co-additives in combination with such clarifiers to achieve certain benefits, none of the prior art references known to date have disclosed a method and composition that may be employed to improve the clarifying function of such clarifiers at relatively low loadings, namely, to improve the low level efficacy of these clarifiers.
In some applications, low levels of such clarifiers are highly be desirable. For example, it will be more economic to use less clarifier, and it will reduce any possible plate out, blooming, or extraction. A method, process, or chemical composition that makes it possible to improve or increase the low level efficacy of nucleating or clarifying agents is highly desirable. The current invention is directed towards such improvements.